How do you determine that a particle is/was entangled?

In summary, the thought experiment suggests that if there exists a physical behavior that is repeatable and predictable, then there should be 'possible' knowledge of such an existence. However, there is no deterministic principle nor basis theory why communicative operators must stop there.
  • #36
JesseM said:
http://grad.physics.sunysb.edu/~amarch/PHY5655.gif [Broken]

http://grad.physics.sunysb.edu/~amarch/ [Broken]
http://grad.physics.sunysb.edu/~amarch/Walborn.pdf [Broken]

Nice, experiment.

Note however that all results are just what one would expect from classical optics...


If you look at the pdf.

FIG 2: The interference pattern.

FIG 3: The interference pattern disappears after the insertion of the quarter-wave plates.

The pattern also disappears in classical optics: The Left plus Right hand polarized light
produces TWO interference patterns. One for horizontal and one for vertical polarized light.
The two patterns are 180 degrees displaced relative to each other and together they
sum up to the "non-interference" pattern of Figure 3.


FIG 4: The interference pattern reappears if a 45 degrees polarizer is inserted in the other beam.

This is actually one of the two interference patterns mentioned above. The polarizer
absorbes ~70% of the photons and 45 degrees polarized photons have the highest
change of getting through.

Therefor the -45 degrees photons at the dual split side have the highest change of
getting counted. What does happen now at the quarter-wave plates? Look at the table
in the middle of http://grad.physics.sunysb.edu/~amarch/ [Broken]

Code:
QWP1:  0 degrees --> Right polarized light, 90 degrees --> Left  polarized light
QWP2: 90 degrees --> Left  polarized light,  0 degrees --> Right polarized light

Under 45 (or -45) degrees they both produce linear polarized light. Either both horizontal
or both vertical light. Figure 4 therefore corresponds with one of the two interference
patterns adding up to the "non-interference" pattern of Figure 3.


FIG 5: A 180 degrees displaced pattern appears if photons which tend to have a -45 degrees polarization are counted.

This is the other of the two interference patterns adding up to Figure 3. The displacement
of the interference pattern is determined by classical optics. Figures 4 and 5 add up to
Figure 3 as they should do.




Regards, Hans
 
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  • #37
Hans de Vries said:
http://grad.physics.sunysb.edu/~amarch/ [Broken]
http://grad.physics.sunysb.edu/~amarch/Walborn.pdf [Broken]

Nice, experiment.

Note however that all results are just what one would expect from classical optics...


If you look at the pdf.

FIG 2: The interference pattern.
But that with no quarter-wave plates, the interference pattern is only visible in the coincidence count of detections at both Ds and Dp as shown in Fig. 2, if you looked at the total pattern of photons at Ds without doing any coincidence-counting you wouldn't see an interference pattern in this case (no non-coincidence graphs are actually shown on that webpage, but this follows from what we'd been discussing earlier on the thread)...would that also be true in classical optics?
 
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  • #38
JesseM said:
But that with no quarter-wave plates, the interference pattern is only visible in the coincidence count of detections at both Ds and Dp as shown in Fig. 2, if you looked at the total pattern of photons at Ds without doing any coincidence-counting you wouldn't see an interference pattern in this case (no non-coincidence graphs are actually shown on that webpage, but this follows from what we'd been discussing earlier on the thread)...would that also be true in classical optics?

Well there are no photons of course in classical optics, but you would get the two interference
patterns of Figure 4 and Figure 5 if you shine either 45 or -45 degrees linear polarized light on the
double split setup including the quarter wave plates, and no interference pattern if the light was
randomly linear polarized.

Regards, Hans
 
  • #39
Hans de Vries said:
Well there are no photons of course in classical optics, but you would get the two interference
patterns of Figure 4 and Figure 5 if you shine either 45 or -45 degrees linear polarized light on the
double split setup including the quarter wave plates, and no interference pattern if the light was
randomly linear polarized.

Regards, Hans
It seems from the pdf that the quarter wave plates are simply the ones described here.

http://en.wikipedia.org/wiki/Wave_plate

A simple birefringent crystal where H and V polarized light travels at different speeds
and exit the cristal with a 90 degrees relative phase shift between the two. This turns
the linear polarized light in circular polarized light depending on the angle.

Waveplate.png


If the linear polarized light is aligned with either the fast axis or the slow axis then
it will stay linear polarized light.

From to the pdf:

Introducing the λ/4 plates one in front of each slit with the fast axes at angles θ1
= 45° and θ2 =-45° to the x direction
This means that, according to classical optics, linear polarized light at 45° or -45° will stay
linear polarized light a 45° or -45° and interference should occur.

In this particular experiment I don't see any deviation from classical optics, unlike the
Wollaston prism experiments where Malus law doesn't predict the outcome for individual
entangled photons.Regards, Hans.
 
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  • #40
Hans de Vries said:
In this particular experiment I don't see any deviation from classical optics, unlike the
Wollaston prism experiments where Malus law doesn't predict the outcome for individual
entangled photons.
But this experiment is all about coincidence counts between photons at one detector and entangled photons at another--it's only in the coincidence counts that the interference effects are seen, the total pattern of light at the Ds detector wouldn't show interference in any phase of the experiment. Since there's no analogue for coincidence-counting of entangled particles in classical optics, and you say that in classical optics the total pattern of light at the position of Ds would show an interference pattern in the setup with the quarter-wave plates, it seems strange to say "I don't see any deviation from classical optics".
 
  • #41
JesseM said:
But this experiment is all about coincidence counts between photons at one detector and entangled photons at another--it's only in the coincidence counts that the interference effects are seen, the total pattern of light at the Ds detector wouldn't show interference in any phase of the experiment. Since there's no analogue for coincidence-counting of entangled particles in classical optics, and you say that in classical optics the total pattern of light at the position of Ds would show an interference pattern in the setup with the quarter-wave plates, it seems strange to say "I don't see any deviation from classical optics".


There's a very distinct difference between the usual EPR experiments with a Wollaston prism
and this experiment:

The angle of linear polarization of the photons explains all the results.

This angle is what is used as the "Bohm hidden variable" in other EPR experiments where it
fails to explain the experimental results.


Regards, Hans
 
  • #42
Hans de Vries said:
Well there are no photons of course in classical optics, but you would get the two interference patterns of Figure 4 and Figure 5 if you shine either 45 or -45 degrees linear polarized light on the double split setup including the quarter wave plates, and no interference pattern if the light was randomly linear polarized.

This cannot be accurate, because classical optics does not provide a theory of entangled waves either. That is what we would need to model - where is that model? Is there a reference?

And yet, ordinary EPR experiments rule all such models out a priori! So trying to model a classical explanation of a Quantum Eraser experiment fails before we get started. On the other hand, all QM-based models must be able to explain/predict the results of these experiments... and they do.

So I completely disagree with any statement to the effect that a classical model can explain the series of results in the reference supplied above by JesseM.
 
  • #43
DrChinese said:
So I completely disagree with any statement to the effect that a classical model can explain the series of results in the reference supplied above by JesseM.

Well that's very outspoken. Let's study the experiment then and go through it step by step:

Classical optics tells us that:

1) +45° linear polarized light will produce an interference pattern.
2) -45° linear polarized light will produce an interference pattern displaced by 180°
3) A mix of arbitrary linear polarized light will produce no interference pattern

Why do you think this is wrong from a classical optics point of view?

Regards, Hans
 
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  • #44
Hans de Vries said:
1) +45° linear polarized light will produce an interference pattern.
2) -45° linear polarized light will produce an interference pattern displaced by 180°
3) A mix of arbitrary linear polarized light will produce no interference pattern
+45° polarized light is on the fast axis of one birefringent quarter-wave plate and on
the slow axis of the other quarter-wave plate. It sees only a single refraction index per
quarter wave plate. The lower one on the first and the higher one at the second.

The photon stays +45° linear polarized at both quarter-wave plates and an interference
pattern occurs. The only difference due to the quarter wave plates are the absolute and
relative phase shifts, introduced by the propagation through the plates, which can shift
the interference pattern.


-45° polarized light is on the slow axis of one birefringent quarter-wave plate and on
the fast axis of the other quarter-wave plate. This means a +90° degrees phase shift
on the path of the first one and a -90° degrees phase shift on the path of the second
one. The total relative phase shift is 180° which causes the entire interference pattern
to shift by 180° Regards, Hans
 
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  • #45
Hans de Vries said:
There's a very distinct difference between the usual EPR experiments with a Wollaston prism
and this experiment:

The angle of linear polarization of the photons explains all the results.

This angle is what is used as the "Bohm hidden variable" in other EPR experiments where it
fails to explain the experimental results.


Regards, Hans
But classical linear polarization can have a continuous set of angles, while the linear polarization detectors always give one of two results (labeled x and y on the chart in the 'Which-Way Marker' section of http://grad.physics.sunysb.edu/~amarch/ [Broken]), and likewise the circular polarization detectors always give one of two results (labeled L and R in that chart). If you're assuming the linear polarization angle is the hidden variable for the s photons, what are you assuming about the relationship between that variable and the result obtained by these two detectors? For a given linear polarization angle on an s photon, how do we decide whether the entangled p photon will give result x or result y? And for that same linear polarization angle, how do we decide whether the s photon will pass through slit 1 or slit 2 with the quarter wave plates in place? One thing that's clear from the chart is that if the entangled p photon gave result x, then if the s photon goes through slit 1 it must be measured to have circular polarization R and if it goes through slit 2 it must be measured to have circular polarization L, whereas if the p photon gave result y then this is reversed.
 
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  • #46
Surely an entangled photon cannot affect or 'act on' its partner, it is only correlated and even that is only a synchronisation of state observables wrt time. If bob is observed as an x then alice instantly becomes a y?
 
  • #47
Hans de Vries said:
http://grad.physics.sunysb.edu/~amarch/ [Broken]
Under 45 (or -45) degrees they both produce linear polarized light. Either both horizontal
or both vertical light. Figure 4 therefore corresponds with one of the two interference
patterns adding up to the "non-interference" pattern of Figure 3.

Regards, Hans


Exactly, and there are some claims that lack of interference pattern is due to 'which way' information whereas its due to two normal interference patterns that have been added.
 
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  • #48
JesseM said:
But classical linear polarization can have a continuous set of angles, while the linear polarization detectors always give one of two results (labeled x and y on the chart in the 'Which-Way Marker' section of http://grad.physics.sunysb.edu/~amarch/ [Broken]), and likewise the circular polarization detectors always give one of two results (labeled L and R in that chart). If you're assuming the linear polarization angle is the hidden variable for the s photons, what are you assuming about the relationship between that variable and the result obtained by these two detectors? For a given linear polarization angle on an s photon, how do we decide whether the entangled p photon will give result x or result y? And for that same linear polarization angle, how do we decide whether the s photon will pass through slit 1 or slit 2 with the quarter wave plates in place? One thing that's clear from the chart is that if the entangled p photon gave result x, then if the s photon goes through slit 1 it must be measured to have circular polarization R and if it goes through slit 2 it must be measured to have circular polarization L, whereas if the p photon gave result y then this is reversed.

The point is that they do not use the standard interference rules for a massless vector field
where components under 90° do not interfere (H and V). Instead they seem to confuse it
with the interference rules for a spinor field where spin-up and spin-down states do not
interfere.

The classical EM field rules are good enough to determine the interference pattern since
the QED propagator for the photon field (1/q^2) is classical. A (more or less) 50:50 mixture
of L and R circular polarized radiation gives linear polarized radiation so one should not
expect to measure circular polarized photons and be able to obtain which-way information.Regards, Hans
 
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  • #49
Cthugha said:
True, but this will still not tell you, whether there is no interference pattern due to entanglement or whether the light is just extremely incoherent.

You can view that extremely incoherent beam as "entangled with the environment". I think there is some virtue in viewing entropy as entanglement with the environment.
 
  • #50
Hans de Vries said:
Well that's very outspoken. Let's study the experiment then and go through it step by step:

Classical optics tells us that:

1) +45° linear polarized light will produce an interference pattern.
2) -45° linear polarized light will produce an interference pattern displaced by 180°
3) A mix of arbitrary linear polarized light will produce no interference pattern

Why do you think this is wrong from a classical optics point of view?

Regards, Hans


Classical optics does not feature entangled waves. Yet wave entanglement can be demonstrated - in an EPR setup, the waves must show perfect correlation. Provide an explanation of that behavior and we will have a good starting point to discuss the next hurdle.

Obviously, we are simply heading down the path of Bell... and we already know where that leads. So that is why I made my "bold" statement. It does not make sense to offer a model that has already been experimentally excluded. No quantum eraser model is going to be viable for a model which cannot pass a Bell test first.

I am not trying to be contrary, just saying that you cannot ignore Bell in this situation.
 
  • #51
LaserMind said:
Surely an entangled photon cannot affect or 'act on' its partner, it is only correlated and even that is only a synchronisation of state observables wrt time. If bob is observed as an x then alice instantly becomes a y?

I would say that is the question, and the answer is not known. But you cannot assume this to be true.
 
  • #52
DrChinese said:
I would say that is the question, and the answer is not known. But you cannot assume this to be true.
Dr C -
hmmm, can you give me a reference(s) to experiments or theorems that indicate otherwise? Sounds like a UFO hunt to me, but I am eager to dig into it and change my opinion.
 
  • #53
LaserMind said:
Dr C -
hmmm, can you give me a reference(s) to experiments or theorems that indicate otherwise? Sounds like a UFO hunt to me, but I am eager to dig into it and change my opinion.
You could take a look at Bohmian mechanics, an interpretation of QM which makes all the same physical predictions as any other version of QM, but which features hidden variables that can influence one another instantaneously at a distance.
 
  • #54
LaserMind said:
Dr C -
hmmm, can you give me a reference(s) to experiments or theorems that indicate otherwise? Sounds like a UFO hunt to me, but I am eager to dig into it and change my opinion.

JesseM correctly points out Bohmian Mechanics (BM, also referred to as de Broglie Bohm or dBB). In this view, the state of particles that are non-local to the entangled particles themselves provide critical influences that are intended to describe and explain the entanglement mechanism. Some groups of scientists have studied this concept in depth. While the ideas are still imperfect, the basic Bohmian programs are able to reproduce the testable essentials of QM. However, there is no relativistic version yet and there is no "one" version of BM as there are competing versions.

You can also imagine that there might be force carriers, previously unknown and with no other known footprint, which have the ability to travel FTL. Perhaps these are exchanged between entangled particles. This, of course, would be pure speculation and there is no evidence whatsoever to support the view... but it is possible.

Either way, it is *possible* that non-locality is a part of nature.
 
  • #55
May I offer a criticism of the Walborn, Padua experiment? Note carefully that the two terms superposed in equation 2 describe the probable results of a measurement of the compound object, composed of a photon (described by psi) and the which-path marker (described by M). The authors show that results of a measurement of the compound (for emphasis, COMPOUND) system, specified by QM as the absolute square of Psi (that’s capital Psi, the left side of equation 2), include no cross terms, indicative of no interference. Again, carefully now, this means that if one measured the compound system with some as yet unspecified apparatus, that compound system would not exhibit interference. It would have no interference because the eigenfunctions of M are orthogonal.

But it’s not an interference pattern of the compound system that is supposed to disappear and then be restored via quantum erasure. Instead, the interference pattern of a single photon, described by psi, is what we are interested in. It is absolutely not the case that because a compound object exhibits no interference, then its constituent systems will also exhibit no interference. (For simplicity, this is like an exploding artillery shell, composed of fragments. We cannot tell where each fragment went by looking at the center of momentum of the entire shell. The compound object doesn’t tell us everything about each constituent. The single photon wavefunction, psi, determines everything about the photon, not the compound wavefunction, Psi.)

The implied inference the authors give us, that the photon interference is gone, is incorrect, and misleading. It’s no surprise, then, that if the interference never disappeared, it can be made to reappear.

It seems to me that we ought always to keep in mind, when discussing quantum erasure, the original theoretical analysis from 1978 that has generated all these QE experiments over the years. (Sculley, et al., Phys. Rep. 43, p. 485) (Am I actually the only person who has read that article carefully?) Scully and his colleagues meticulously described the quantum mechanics for a heavy molecule of spin one-half traveling through a modifoed Stern-Gerlach magnet. They placed a measuring apparatus, in this case a bi-level atom, in one arm of the magnet. They say that if the molecule goes that way, it will always kick the atom into its excited state, thus measuring which path was taken.

They use Schrodinger’s equation, of course, to show that after passing through the magnet, the density matrix for the molecule-atom system will be almost diagonalized. Meaning that the off-diagonal terms are small compared to terms on the diagonal. (That’s not really a diagonalized matrix, by the way.) Their analysis depends on the crucial assumption that because the molecule is arbitrarily heavier than the atom, there will be an arbitrarily small momentum transfer to the spinning molecule. This, they assert, implies no “significant” change to the molecule’s wavefunction at measurement, so continuous Schrodinger evolution continues, they claim.

But, consider this: no matter how heavy the molecule is, it always kicks the atom from ground to its excited state. That’s a definite quantized energy. Energy is conserved, so the molecule always loses that same quantum of energy. Each distinct, total energy state of the molecule is specified by a unique, linearly-independent eigenfunction. Each such eigenfunction specifies a distinct, independent, measured state of the molecular system. So, this claim we often hear, that the measurement did not disturb the object measured, is not justified by this analysis.

Scully et al. then imply that since the molecule’s wavefunction evolved continuously through measurement, we ought to be able to reverse that evolution by reversing (erasing) the state of the atomic detector. That’s what was meant, initially, by quantum erasure. Thus, disappearance of interference, evidence for a measurement, is supposed to be restored by returning the apparatus to its ground state. But it’s not a credible physical theory.

I realize that those who’ve advocated for quantum erasure have changed its meaning over the years, as successive experiments have proved unpersuasive. We now hear of object-apparatus entanglement, and sub-ensemble sorting. But, if the theory is not consistent and comprehensible, its not scientifically sound.

DocMike
 
  • #56
Dbar_x said:
I realize that those who’ve advocated for quantum erasure have changed its meaning over the years, as successive experiments have proved unpersuasive. We now hear of object-apparatus entanglement, and sub-ensemble sorting.
Object-apparatus entanglement? Where do we "now hear" of that? If you're referring to my posts on the "Interference seen in a member of an entangled pair" thread, you didn't read very carefully: I made it quite clear that object-apparatus entanglement would only be necessary to analyze a wholly impractical thought-experiment where a macroscopic apparatus can remain completely isolated from the environment for a long period of time (long enough that if you measure it at an earlier time you will gain which-path information, but if you measure it at a later time it will give you no which-path information), akin to the Schroedinger's cat thought-experiment. In practical quantum eraser experiments, the "marker" that has the potential to give you which-path information (unless it is measured in such a way that this information is 'erased', or if you prefer, never existed, which as I said in post 56 here is just a semantic issue) is just another entangled particle, like the "idler" that can give you which-path information for the signal photon in the delayed choice quantum eraser. There is no need here to imagine that the measuring apparatus becomes entangled with what it measures, the assumption that measurements collapse the 2-particle wavefunction should work just fine. If you know a practical quantum eraser experiment where the authors felt the need to assume the measuring system becomes entangled with the particle being measured, please point it out.
 
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  • #57
Object-apparatus entanglement? Where do we "now hear" of that?

Mohrhoff's article in the American Journal of Physics in 1996 was the most pointed criticism of quantum eraser theory that I know of. The response of Scully and his colleagues was, it seems, to find a different meaning for quantum erasure. They did some calculations, not entirely persuasive, which led to an explanation involving entanglement and the sorting of coincidence measurements into sub-ensembles. One sub-ensemble correlated with interference, the other sub-ensemble with no interference.

DocMike
 
  • #58
Dbar_x said:
Mohrhoff's article in the American Journal of Physics in 1996 was the most pointed criticism of quantum eraser theory that I know of. The response of Scully and his colleagues was, it seems, to find a different meaning for quantum erasure. They did some calculations, not entirely persuasive, which led to an explanation involving entanglement and the sorting of coincidence measurements into sub-ensembles. One sub-ensemble correlated with interference, the other sub-ensemble with no interference.
What do you mean by "sub-ensembles" in this context? And what does this have to do with entanglement between the particles and the measuring apparatus? If these papers are not available online, perhaps you could quote a relevant paragraph or two?

edit: The ensemble interpretation of QM interprets the wavefunction as just giving statistical predictions about experimental results on an ensemble of trials where the system is prepared in the same initial state and measured the same way. So are sub-ensembles just the theoretical analogue of a coincidence count, like the probability the signal photon will be detected at different points on the screen given the assumption that we're looking at the subset of trials where the idler was detected at, say, detector D2?
 
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  • #59
Wheelers delayed path does not require backwards in time theories provided we use the CI approach. If its presented as a typical wave particle conundrum then all types of horrors emerge (read Wheeler's analysis) - I am expecting the same of the Quantum Erasure claims by Scully and am requesting a re-evaluation.
 

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